[Doc] Fill API sphinx docs page

Author: aobolensk

docs/user_guide/api.rst

@@ -4,5 +4,26 @@ API Reference
 .. toctree::
    :maxdepth: 1
 
-.. .. doxygenindex::
-..    :project: ParallelProgrammingCourse
+Runners Module
+--------------
+
+.. doxygennamespace:: ppc::runners
+   :project: ParallelProgrammingCourse
+
+Task Module
+-----------
+
+.. doxygennamespace:: ppc::task
+   :project: ParallelProgrammingCourse
+
+Utility Module
+--------------
+
+.. doxygennamespace:: ppc::util
+   :project: ParallelProgrammingCourse
+
+Performance Module
+------------------
+
+.. doxygennamespace:: ppc::performance
+   :project: ParallelProgrammingCourse